Bird Identification: Bird Names
So far when I have referred to a bird I have given it its English name, but I have not explained what a ‘species’ is or indeed the need for Latin or scientific names. As birdwatchers, we are chiefly interested in species, whether it is a Robin, which has the Latin or scientific name of Erithacus rubecida, or a Great Tit with its scientific name Partis major. A species is a population or group which can interbreed and produce fertile offspring. If a member of one species attempted to breed with a member of another species it would generally fail unless the two species were very closely related; if they did succeed in mating then any offspring they produced would be infertile. The species is something real. It is a definite biological unit, which is committed to its own line of evolution.
A species may have exactly the same characteristics throughout the whole of its geographical range or it may consist of populations which vary in appearance, size, song and even habitat. An example of the first group, known as monotypic, is the Osprey, which has a worldwide distribution.
The other group, with slightly differing populations, is known as polytypic. Sometimes these populations become sufficiently distinct from other groups of the same species that they are known as geographical races or subspecies, which, should they meet and interbreed could still produce fertile offspring. If, however, the populations remain isolated for a sufficiently long period, they might develop in such a way that they can no longer produce fertile offspring and can, therefore, be regarded as distinct species. One of the best examples of a polytypic species whose ‘end-forms’ are now distinct species is provided by the Herring Gull. If you follow the different races of the Herring Gull from western Europe round the globe through northern Africa, north-east Russia and back to Europe you will find that there is a gradual and continuous change.
Finally in Europe the two forms overlap, and the second form is known to us as the Lesser Black-backed Gull. Herring Gulls no longer generally interbreed with Lesser Black-backed Gulls or, if they do, they do not usually produce fertile offspring. Both species nest on the Pembrokeshire island of Skokholm, and occupy different parts of the island, the Herring Gull nesting on the cliff face and the Lesser Black-backed Gull on the plateau area of the island. Furthermore, the courtship of the two species has in the course of time changed sufficiently so that it is difficult for the two species to interbreed, and they are not only isolated by habitat but, by and large, reproductively isolated too. The Lesser Black-backed Gull is migratory while the Herring Gull is not. This story of the two gulls is a very good example of how geographical isolation – or perhaps distance isolation – can lead to the formation of new species. In this case, each step in the change of the characteristics was so distinct as to be regarded as a race or subspecies. Where finally they overlapped they were geographically, ecologically and to some extent reproductively isolated.
In other cases the changes of colour or size may be very gradual and no distinct step is recognizable; this sort of graduation is known as a ‘cline’. The best known example of this is shown by the Guillemot. Some of these birds have a white ring round the eye with a white line over the ear coverts. This mark looks rather like a pair of spectacles and the birds which have this mark are known as bridled Guillemots. The interesting point is that the more northerly population of Guillemots has a higher percentage of bridled forms than those in the south of England. Mr H.N. Southern has organized three surveys at roughly ten-year intervals to discover the percentage of bridled Guillemots in the populations in different parts of the British Isles. While the percentages have changed slightly over the years Southern’s figures have shown that less than 1% are bridled in the south of England, 26% are bridled in Shetland, 34% in the Faeroes and 50-70% are bridled in southern Iceland.
The species, then, is a distinct biological unit. Taxonomists have grouped together the species which they think have similar appearances and characteristics into what are known as ‘genera’ but this grouping is purely subjective.
Linnaeus, a Swedish zoologist, devised a system of naming animals in his Systerna Naturae in 1758. It is known as the binomial system, every species is placed in a genus and the scientific name of the species thereafter consists both of the generic and the specific names. Hence when I refer to the scientific name of the Blackbird and Song Thrush as Turdus merula and Turdus philomelus I am also indicating that those birds have been put in the same genus, Turdus, with the specific names of merula and philomelus.
It so happens that when studying evolution, systematists wish to give names to the geographical races that I have mentioned earlier in relation to the Herring Gull and Lesser Black-backed Gull. They have in fact gone one stage further and produced the trinomial system. The generic and specific names remain the same but the subspecies is given a third name. The Herring Gull, for instance, which occurs in Britain is Larus argentatus argentatus, whereas the subspecies which occurs in the United States is Larus argentatus smithsonianus. A word of warning: the identification of geographical races in the field is very difficult.
Above the level of species and genus again taxonomists tend to differ over certain points. There are many divisions and sub-divisions, but here I will only mention three: family, order and class. A family is a grouping of genera which taxonomists expect to have evolved from the same parent stem. For instance, the family Fringilladae includes not only finches like the Chaffinch and Linnet but also the Crossbill. An order is a much broader division within the class. The aim of an order is to express the relationship between families of a similar origin. The families Corvidae (crows), Paridae (tits) and Emberizidae (buntings) are all part of the order Passeriformes. However, some of the relationships are not always clear and are in dispute. Finally, all birds belong to the class Aves.
Nowadays, most lists of birds are set out in what is known as the Wetmore Order, named after the American zoologist Dr Alexander Wetmore who published his proposals in 1960. In making lists of birds for identification books and field guides it is usual to begin with what are believed to be the most primitive forms of birds and work towards the most highly developed. However, it must be realized that all orders listed in this form have no certainty and it is difficult to express any linear relationship between birds. Professor K. H. Voous has just published a List of recent ho/arctic bird species (roughly that part of the globe north of the Tropic of Cancer), and this will no doubt be the standard list for some time to come and has been followed by the editors of the Handbook of the Birds of Britain, Europe and North Africa. For the person who is not concerned about taxonomy there is a growing tendency to list birds within their genera in the alphabetical order of their specific names.